The overall objective of this project is to determine whether treatment of acute hyperammonemia with N-carbamyl-L-glutamate (NCG) in propionic acidemia (PA), methylmalonic acidemia (MMA), carbamyl phosphate synthetase 1 deficiency (CPSD) and ornithine transcarbamylase deficiency (OTCD) changes the clinical outcome of disease. The specific aims are: 1. To determine whether NCG treatment of acute hyperammonemia in severe, neonatal-onset PA and MMA improves neurodevelopmental outcome, and whether it is safe. 2. To determine whether NCG treatment of acute hyperammonemia accelerates the resolution of hyperammonemia and clinical recovery in patients with severe PA and MMA and in those with partial CPSD and OTCD, and whether it is safe. 3. To determine whether, in metabolically stable patients, the effect of a 3-day NCG treatment on ureagenesis is predictive of the outcomes observed in Aims 1 and 2. This is a double blind, placebo controlled trial performed in seven children's hospitals across the country. The primary outcome measure for Aim 1 is neurocognitive development assessed using age-appropriate specific neurodevelopmental instruments. Primary outcomes of Aim 2 include the effect of NCG on the pace of resolution of hyperammonemia and its clinical symptoms and the length of hospitalization. The Aim 3 outcome is an assessment of the correlation between the short-term effect of NCG on ureagenesis rate and the clinical response to NCG long term and in acute clinical settings as determined by Aims 1 and 2. Our hypothesis is that NCG will improve clinical outcome in these disorders. These studies are designed to provide scientific evidence for NCG efficacy and safety which can be used to expand the indication of this drug, leading to FDA approval.